Serologic surveillance of rubella vaccinees and of natural immunes to determine the long term persistence of antibodies in the two groups will be continued. Immune responses to rubella virus antigens will be further characterized, with emphasis on differences between protective neutralizing antibodies acquired through vaccination or as a result of natural infection. Laboratory studies will focus on a) identification of subclasses of IgG with individual rubella virus antigens; b) use of specific antisera against individual viral proteins to identify the antigens which participate in neutralization; and c) development of a radioimmune assay for the measurement of NT antibody, utilizing purified monospecific reagents.